No file output after rendering?
As an extension of Keavon's answer, Blender does not write still renders to the hard drive by default in the GUI. Command line renders are written to the hard drive automatically, and there is an addon which adds this functionality in the GUI. (see this post).

There are a few ways to work around this:


Render a one frame animation as in Keavon's answer.
Save the image manually from the UV/Image editor after the render is complete (press F3 with the cursor in the image editor, or select the option from the header):



For 2.8 Use the Image Editor, and select Image > Save As (or press Shift+S)



You will be taken to the File Browser with options to set the format and location of the saved image:


Render by running bpy.ops.render.render(write_still=True) in the Python Console. The render will be written to the output path.
Use a File Output node as mentioned by Matt:



This will create a file /tmp/test.png. Note that the File output node has format and output path settings in the Properties region (N) which are separate from the scene settings.